Most liquid crystal displays installed in a mobile device typified by a mobile phone are of a transmissive type or a semi-transmissive type that require backlight. In the transmissive or semi-transmissive liquid crystal display, however, it has been known that display image information is negated by intense external light, for example, under high illuminance environment such as sunlight and the visibility thereof is lowered. On the other hand, in a reflective liquid crystal display which has good visibility under sunlight, the amount of light to be the basis of the display image information is insufficient under low illuminance environment, for example, indoors and the visibility thereof is lowered.
As a technique for improving the visibility in a transmissive or semi-transmissive liquid crystal display, there is a method disclosed in Japanese Patent Application Laid-Open Publication No. 2001-265463. In this method, means for controlling display brightness of a display device in accordance with illuminance information obtained by measuring and detecting environmental illuminance by an external light sensor and means for controlling backlight illuminance of a keyboard are provided. In the control described in Japanese Patent Application Laid-Open Publication No. 2001-265463, for example, under intense illuminance environment such as sunlight, in order for display image information not to be negated by external light, the amount of backlight is increased to improve visibility of a liquid crystal display. On the other hand, under low illuminance environment such as indoors, since the amount of backlight is relatively large and there are few factors by which display image information is negated, the amount of backlight is not increased or is reduced. By theses controls, it becomes possible to increase the amount of backlight outdoors to give priority to visibility and reduce the amount of backlight indoors to give priority to power consumption. By this means, both of low power consumption and high visibility of a liquid crystal display can be realized.
Incidentally, there are mainly two types of external light sensors. One is a linear output illuminance sensor in which an output illuminance signal has a linear relationship with the amount of incident light, and the other is a logarithmic output illuminance sensor in which an output illuminance signal has a logarithmic relationship with the amount of incident light. Since a relationship between incident light illuminance and output current is linear in a photodiode used for a light sensor, the former linear output illuminance sensor was mainstream previously. In recent years, however, the logarithmic output illuminance sensor capable of detecting illuminance difference finely in a low illuminance range has appeared, and the current situation is that these two types of illuminance sensors are present in a mixed manner. Further, in the aforementioned logarithmic output illuminance sensor, it has been known that input-output characteristics of the sensors are different among manufacturers or products.
The targets to be achieved by the present invention are to make it possible to control the amount of backlight in accordance with an output of a sensor even in various types of external light sensors such as a linear output illuminance sensor and a logarithmic output illuminance sensor and further to make it possible to perform similar control of the amount of backlight even when the same type of sensor different in input-output characteristics is used.
An object of the present invention is to provide a display device such as a liquid crystal display with good visibility and low power consumption by combining backlight control means and various types of external light sensors.